Aetiology

Abusive head trauma occurs when an infant is violently shaken, causing back and forth, and rotational movement of the head. The injuries seen can result from a fairly short period (5-10 seconds to <1 minute) of shaking alone.[14][15]​ Children may also sustain abusive head trauma subsequent to inflicted blunt force trauma to the head or an inflicted crushing injury.

It is hypothesised that crying is the most common trigger to the shaking of an infant. Studies of perpetrator confessions support this hypothesis, revealing that crying is the most common antecedent to the loss of control and violent shaking of a child.[12][14][16]

Pathophysiology

Infants are especially susceptible to abusive head trauma due to their large head-to-body ratio, their weak neck muscles, the relative lack of myelination of nerves when compared with older children, and the disparity in size between victim and perpetrator.

Violent shaking subjects an infant’s brain to repetitive, rotational force. This triggers a cascade of events leading to cranial and ophthalmological pathology.

Subdural haemorrhage is a common finding and occurs from direct trauma, or bleeding from bridging vessels that are torn when the brain moves inside the skull. The arachnoid may also tear, leading to leakage of cerebrospinal fluid into the subdural spaces, causing the appearance of mixed-density subdural haemorrhage on computed tomography (CT) or magnetic resonance imaging (MRI).[17]

Neuronal damage occurs from blunt force trauma (described as an acceleration-deceleration injury), shearing injuries, and secondary damage from excitatory amines, which are released by dying brain cells. There is loss of cerebral autoregulation and disruption of ionic homeostasis.[18] In many cases, it is felt that damage to the craniocervical junction leads to apnoea and hypoxic brain injury.[18] Brain tissue injury often occurs at the junction of areas of differing density (such as the grey-white junction), and infants who are shaken appear to have greater injury depth than those with accidental injury.[19]

The forces that result from blunt force trauma are much higher than forces from shaking alone; thus, some clinicians believe that in children with severe brain injury there was likely not only shaking, but also direct head trauma.

Ophthalmological findings, which may be present in cases of violent shaking, are hypothesised to result from vitreo-retinal traction that can lead to numerous multi-layer, widespread retinal haemorrhages that may be bilateral or, in some cases, unilateral. There may also be retinoschisis (splitting of the layers of the retina), optic nerve sheath haemorrhage (observed at post-mortem examination), and vitreous or conjunctival haemorrhage.[20]

Abusive injury can lead to scalp and skull injury, including visible or subcutaneous bruising of the scalp, neck muscle haemorrhage, and skull fracture.[1]

In addition, some infants may have bruising, fractures, or intra-abdominal injury related to concomitant physical abuse. Rib fractures from squeezing of the chest and metaphyseal corner fractures of the long bones from trauma are common, but not necessary to make the diagnosis.

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